1. Field of the Invention
The present invention relates to a signal transmission apparatus provided with a flat cable and lead wires with which standardized conductors for the flat cable can be used universally.
2. Description of the Related Prior Art
In a steering signal transmission apparatus a vehicle, signals from a flat cable (hereafter also referred to as an FFC) wound around a rotating body is transmitted to the outside via electrical wire.
The steering signal transmission apparatus shown in FIGS. 5, 6A-6C is disclosed in Japanese Utility Model Application Laid Open Publication No. 1992/24611. Reference number 51 indicates a first housing which constitutes a stator and reference number 52 indicates a second housing which constitutes a rotor. The two housings are fitted together concentrically one into the other so that they can rotate relative to each other, and a flat cable 54 is coiled inside a ring-like chamber 53 formed between the two housings. One end of the flat cable 54 is secured to the first housing and the other end is secured on the side of the second housing 52.
Conductors 54a at one end of the flat cable 54, are each connected to one end of a joint bar 55 by welding. A plurality of joint bars 55 are linked together in advance with an insulating supporting body 56 in correspondence to the pitch of the conductors 54a of the flat cable 54 and, at the other end of each joint bar 55, a corresponding lead wire 57 is connected by welding. The connecting portions of the individual conductors connected to these joint bars 55 are protected as a unit by a resin molded protective body 58, which, at the two sides, partially includes insulating coverings 54b of the flat cable 54 and 57b of the various lead wires 57.
However, when an air bag is mounted in the vehicle at present, there are specifications that must be conformed to in regard to the electrical resistance value of an air bag circuit of the steering signal transmission apparatus described above, stipulating upper and lower limits, which are imposed by various manufacturers depending upon vehicle models. Because of this, the width w and the thickness t of the flat cable in FIG. 4 are adjusted to set the electrical resistance of the air bag circuit at a specific value. In other words, new conductors must be produced for every manufacturer or vehicle model in which the air bag is to be mounted, necessitating that several types of flat cables be prepared. There is another problem in that the durability and the like of the FFC used in the air bag circuit must be verified for each case.